Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by a progressive deterioration in motor and cognitive function, and its primary genetic cause is the expansion of a polyglutamine (polyQ) tract near the N-terminus of the ubiquitously-expressed, cytosolic protein huntingtin (Htt). Mutant Htt adopts a cytotoxic gain-of-function that causes it to polymerize into insoluble, amyloid-like fibrillar aggregates that are initially deposited in medium spiny neurons in the striatum of the basal ganglia but in later stages of disease, spread to neurons in neighboring brain regions (e.g. the hippocampus and cortex). While it is clear that deposition of Htt aggregates correlates with neuronal cell death, the exact mechanism by which they contribute to cytotoxicity or spread through brain tissue is currently unknown. Recent studies suggest that extracellular Htt aggregates have prion-like properties; i.e. they can trigger the templated conformational change of soluble monomers and transmit the misfolded state to naove cells. Further, emerging evidence suggests that aggregates formed by proteins associated with other neurodegenerative disorders (e.g. Alzheimer's and Parkinson's diseases) also adopt prion-like behavior, suggesting that progression of these diseases may occur by a common mechanism. The overall aim of this proposal is to establish a Drosophila model to demonstrate whether prion-like transmission of HD pathology can be recapitulated in vivo and to explore the mechanism(s) involved in this process. Specific Aim 1 investigates whether the aggregation phenotype can be transmitted from neurons expressing aggregation-prone Htt containing a pathogenic polyQ tract to synaptically-connected neurons expressing soluble Htt with a non-pathogenic polyQ tract. Specific Aim 2 explores an alternative but not necessarily mutually exclusive possibility, that Htt-induced aggregation is transmitted between adjacent, non-synaptically-connected cells. These studies will exploit detailed knowledge of Drosophila olfactory neuronal circuitry and various genetic tools to express pathogenic and non-pathogenic Htt proteins in distinct cell types within an individual fly. The focus of Specific Aim 3 is to identify determinants for the lateral transmission of Htt-induced aggregation in a Drosophila model to gain insight into the mechanism(s) involved in this process. These studies will employ a loss-of-function screen in which genes encoding cell surface and secreted proteins are depleted by RNA interference, and effects will be measured by examining how the spread of Htt-induced aggregation to synaptically-connected and/or neighboring cells is altered. Together, these approaches will expand our current understanding of how Htt aggregates propagate between cells with prion-like properties in an in vivo setting, and will lend insight into the mechanism by which the pathology of HD and other neurodegenerative disorders progress along stereotypical paths through the brains of afflicted individuals.